In the present work, using the method of a composite piezoelectric oscillator at a frequency of 99-102 kHz, polycrystalline magnesium was studied under two types of treatment: deformation and subsequent annealing. The influence of these types of treatments on changes in the dislocation density and such an important structurally sensitive parameter as the dynamic Young's modulus has been established. The values of micro-yield stress are determined. With a longitudinal deformation of 1.7%, at the strain amplitude of 5×10-6 , its value decreased from 2.9 MPa to 1.7 MPa; after annealing at 400 °C, it increased to 5.8 MPa.